No duplicate keys test, fixes #2643 (#2647)

* moved unit tests to conditional compile

made module `tests` and conditionally compiled

* added test to check for duplicate keys in builtin

checks all pairs in GlobMatcher::mappings via `GlobMatcher::glob`s `Eq`
panics at first match

```rs
SyntaxMapping {
  mappings: Vec<(
    GlobMatcher {
      glob: Glob, ...,
    }, ...,
  }, ...,
}
```

* output more info with test failure

* dont duplicatively add git config path

* use guard in match for adding builtin git config syntax mapping

---------

Co-authored-by: Orion Yeung <11580988+orionyeung001@users.noreply.github.com>

src/syntax_mapping.rs

@@ -162,17 +162,24 @@ impl<'a> SyntaxMapping<'a> {
 
         // Global git config files rooted in `$XDG_CONFIG_HOME/git/` or `$HOME/.config/git/`
         // See e.g. https://git-scm.com/docs/git-config#FILES
-        if let Some(xdg_config_home) =
+        match (
-            std::env::var_os("XDG_CONFIG_HOME").filter(|val| !val.is_empty())
+            std::env::var_os("XDG_CONFIG_HOME").filter(|val| !val.is_empty()),
-        {
+            std::env::var_os("HOME")
-            insert_git_config_global(&mut mapping, &xdg_config_home);
-        }
-        if let Some(default_config_home) = std::env::var_os("HOME")
                 .filter(|val| !val.is_empty())
-            .map(|home| Path::new(&home).join(".config"))
+                .map(|home| Path::new(&home).join(".config")),
-        {
+        ) {
-            insert_git_config_global(&mut mapping, &default_config_home);
+            (Some(xdg_config_home), Some(default_config_home))
+                if xdg_config_home == default_config_home => {
+                insert_git_config_global(&mut mapping, &xdg_config_home)
             }
+            (Some(xdg_config_home), Some(default_config_home)) /* else guard */ => {
+                insert_git_config_global(&mut mapping, &xdg_config_home);
+                insert_git_config_global(&mut mapping, &default_config_home)
+            }
+            (Some(config_home), None) => insert_git_config_global(&mut mapping, &config_home),
+            (None, Some(config_home)) => insert_git_config_global(&mut mapping, &config_home),
+            (None, None) => (),
+        };
 
         fn insert_git_config_global(mapping: &mut SyntaxMapping, config_home: impl AsRef<Path>) {
             let git_config_path = config_home.as_ref().join("git");
@@ -242,6 +249,9 @@ impl<'a> SyntaxMapping<'a> {
     }
 }
 
+#[cfg(test)]
+mod tests {
+    use super::*;
     #[test]
     fn basic() {
         let mut map = SyntaxMapping::empty();
@@ -287,3 +297,28 @@ fn builtin_mappings() {
             Some(MappingTarget::MapToUnknown)
         );
     }
+
+    #[test]
+    /// verifies that SyntaxMapping::builtin() doesn't repeat `Glob`-based keys
+    fn no_duplicate_builtin_keys() {
+        let mappings = SyntaxMapping::builtin().mappings;
+        for i in 0..mappings.len() {
+            let tail = mappings[i + 1..].into_iter();
+            let (dupl, _): (Vec<_>, Vec<_>) =
+                tail.partition(|item| item.0.glob() == mappings[i].0.glob());
+
+            // emit repeats on failure
+            assert_eq!(
+                dupl.len(),
+                0,
+                "Glob pattern `{}` mapped to multiple: {:?}",
+                mappings[i].0.glob().glob(),
+                {
+                    let (_, mut dupl_targets): (Vec<GlobMatcher>, Vec<MappingTarget>) =
+                        dupl.into_iter().cloned().unzip();
+                    dupl_targets.push(mappings[i].1)
+                },
+            )
+        }
+    }
+}